1. Field of the Invention
The present invention relates to an optical film and its production method, a polarizer and a liquid crystal display device.
2. Description of the Related Art
As capable of saving power energy consumption and capable of being thin-walled, liquid crystal display devices are widely employed as image display devices such as TVs, personal computers, etc. The liquid crystal display device comprises a polarizer arranged on both sides of the liquid crystal cell therein, in which the polarizer comprises a polarizing film having iodine or dye adsorbed and aligned therein and sandwiched between transparent resin layers put on both sides thereof. In this, the transparent resin layers act to protect the polarizing element, for which a cellulose ester film is used most generally.
With the recent popularization of such liquid crystal display devices, much desired are further thin-walled, large-sized and high-performance devices. A cellulose ester film widely used as a polarizer protective film has a high transmittance, and by dipping in an aqueous alkali solution, its surface is saponified and hydrophilicated to thereby realize excellent adhesiveness to a polarizing element. However, the film has a problem of dimensional change through moisture absorption and water removal in environmental temperature/humidity change. Another problem is that, when the cellulose ester film is incorporated in a liquid crystal display device and when the other constitutive parts of the device that have been deformed through aging degradation or the like therein are kept in contact with the film, display fluctuation often occurs; and the problem has become considered serious with the recent tendency toward advanced demand for body thickness reduction.
For solving the problems, there has been proposed a (meth)acrylic resin film of methyl methacrylate or the like having a small moisture absorption and having a small photoelastic coefficient, as a film that could be substitutable for the cellulose ester film; however, it could not be said that the adhesiveness of the film of the type to a polarizing element could be sufficient, and therefore there still remains a problem in that the film is readily cracked or broken and is brittle (see Patent Reference 1).
As a resin having a higher mechanical strength than existing (meth)acrylic resin such as methyl methacrylate or the like, there has been proposed a (meth)acrylic resin having a lactone ring structure (see Patent References 2 to 5).
However, in case where such an existing (meth)acrylic resin or a (meth)acrylic resin having a lactone ring structure is directly used as a polarizer protective film as it is, then there occurs a problem in that the adhesiveness thereof to a polarizing element is poor. In particular, in case where the (meth)acrylic resin having a lactone ring structure is used as a polarizer protective film directly as it is and when the film surface is processed for adhesiveness enhancement treatment (for example, corona treatment) for the purpose of enhancing the adhesiveness of the film surface with a polarizing element, then there often occurs cohesion failure around the film surface and therefore there occurs a problem in that the adhesiveness of the film surface to a polarizing element is further poorer than that of the existing (meth)acrylic resin.
As opposed to this, there has been proposed a method of using a film that contains a (meth)acrylic resin having a lactone ring structure, as a polarizer protective film, in which a thin layer of a cellulose resin excellent in adhesiveness is formed on a (meth)acrylic resin layer to form a laminate and the adhesiveness of the laminate to a polarizing element is thereby enhanced (see Patent Reference 6).    [Patent Reference 1] JP-A 5-119217    [Patent Reference 2] JP-A 2000-230016    [Patent Reference 3] JP-A 2001-151814    [Patent Reference 4] JP-A 2002-120326    [Patent Reference 5] JP-A 2002-254544    [Patent Reference 6] JP-A 2007-316366